1) Field of the Invention
The present invention relates to a method of and an apparatus for controlling the position of a head for read and write of information with respect to a medium rotated and driven by a spindle having a hydrodynamic bearing.
2) Description of the Related Art
Magnetic disk units are generally used as external memories in the computers. Such magnetic disk units have a head that performs reading or writing of information from or into a magnetic medium (hereinafter, “read-write operation”). A spindle rotates the medium while the head performs the read-write operation. A head position control apparatus controls the position of the head on the medium, while the head performs the read-write operation.
One approach is to use a ball bearing in the spindle. However, the ball bearing produces sound and also increases the non-repeatable runout. One approach to solve these problems is to use a hydrodynamic bearing instead of a ball bearing.
FIG. 13 is a block diagram of a conventional magnetic disk unit. A spindle 12 having a hydrodynamic bearing rotates a medium 13. A motor 11 drives the spindle 12. A head 14 performs the read-write operation with respect to the medium 13. A head position control apparatus 30 controls the position of the head 14 on the medium 13. The head position control apparatus 30 even absorbs the vibrations, which are produced due to rotation of the medium 13, when positioning the head 14 on the medium 13. Such a magnetic disk unit has been disclosed, for example, in Japanese Patent Application Laid-Open No. 08-45169 (see pages 3 and 4, FIGS. 1 and 2).
The vibrations of the medium 13 include RRO of the spindle in a low frequency region, and periodic RRO of the spindle due to the eccentricity of the axis of rotation of the spindle 12 and motor 11. The head position control apparatus 30 compensates for these vibrations by performing an RRO correction.
The RRO correction can be performed by various ways. One approach is low frequency compression. In the low frequency compression, a low frequency component of the RRO of the spindle is compressed using a compression filter. Another approach is amplitude control. In the amplitude control, a repeat control (feedforward control) performed with respect to the periodic RRO of the spindle.
However, the conventional RRO correction has problems in that only the vibrations mentioned above can be compensated. In magnetic disk units, however, a gyroscopic moment is generated when the spindle rotates; and such gyroscopic moment causes vibrations. The conventional RRO correction has a drawback that the vibrations resulting from the gyroscopic moment cannot be eliminated.
Thus, if ball bearing is used, sound is produced and also there is an increase in the non-repeatable runout. On the other hand, if the hydrodynamic bearing is used, there are vibrations, resulting from the gyroscopic moment, that can not be corrected.